# Austenitic stainless steel

Austenitic stainless steel is one of the five families of stainless steel (along with ferritic, martensitic, duplex and precipitation hardened). Its primary crystalline structure is austenite (face-centered cubic). Such steels are not hardenable by heat treatment and are essentially non-magnetic. This structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen. The Incoloy family of alloys belong to the category of super austenitic stainless steels.

## History

During World War 2 the Schaeffler diagram was invented by Anton, who was then a budding metallurgist in the employ of two American welding electrode manufacturers, Harnischfeger Company and A.O. Smith Corporation.

## AISI 200 and 300 series

Austenitic stainless steels are divided into 300-series and 200-series subgroups. In 300 series stainless steels the austenitic structure obtained primarily by adding nickel. In 200 series stainless steels the structure is obtained by adding manganese and nitrogen, with a small amount of nickel content, making 200 series a cost-effective nickel-chromium austenitic type stainless steel.
300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type 304, also known as 18/8 or A2. Type 304 is extensively used in such items as cookware, cutlery, and kitchen equipment. Type 316, also known as A4, is the next most common austenitic stainless steel. Some 300 series, such as Type 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attack (e.g. pitting and crevice corrosion).
Average content by weight (%) of the major alloying elements of most common Cr-Ni austenitic stainless steel grades :
| Euronorm (EN) number | EN designation | AISI grade | C | Cr | Mo | Ni | Others | Melts at | Remark |
| 1.4310 | X10CrNi18-8 | 301 | 0.10 | 17.5 | NS | 8 | NS | 1420 | For springs |
| 1.4301 | X5CrNi18-10 | 304 | < 0.07 | 18.5 | NS | 9 | NS | 1450 | A very common austenitic stainless steel grade |
| 1.4307 | X2CrNi18-9 | 304L | < 0.030 | 18.5 | NS | 9 | NS | 1450 | Similar to the above but not susceptible to intergranular corrosion thanks to a lower C content. |
| 1.4305 | X8CrNiS18-9 $^e$ | 303 | < 0.10 | 18 | NS | 9 | 0.3 | 1420 | Sulphur is added to improve machinability. |
| 1.4541 | X6CrNiTi18-10 | 321 | < 0.08 | 18 | NS | 10.5 | Ti: 5×C ≤ 0.70 | 1425 | Same as grade 1.4301 but not susceptible to intergranular corrosion thanks to Ti which "traps" C. |
| 1.4401 | X5CrNiMo17-12-2 | 316 | < 0.07 | 17.5 | 2.2 | 11.5 | NS | 1400 | Second best known austenitic grade. Mo increases the corrosion resistance. |
| 1.4404 | X2CrNiMo17-12-2 | 316L | < 0.030 | 17.5 | 2.25 | 11.5 | NS | 1400 | Same as above but not susceptible to intergranular corrosion thanks to a lower C content. |
| 1.4571 | X6CrNiMoTi17-12-2 | 316Ti | < 0.08 | 17.5 | 2.25 | 12 | Ti: 5×C ≤ 0.70 | | |
The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series.
Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless. This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid. Alloy 20 has excellent mechanical properties and the presence of niobium in the alloy minimizes the precipitation of carbides during welding.

## Heat resisting austenitic stainless steels

Heat resisting grades can be used at elevated temperatures, usually above 600 °C (1,100 °F).
They must resist corrosion (usually oxidation) and retain mechanical properties, mostly strength (yield stress) and creep resistance.
Corrosion resistance is mostly provided by chromium, with additions of silicon and aluminium. Nickel does not resist well in sulphur containing environments. This is usually taken care of by adding more Si and Al which form very stable oxides. Rare earth elements such as cerium increase the stability of the oxide film.
Typical composition of the major grades :
| EN | EN designation | AISI/ASTM | UNS | C | Cr | Ni | Si | Mn | Others |
| 1.4878 | X8CrNiTi18-10 | 321H | S32109 | < 0.1 | 18 | 10.5 | - | - | Ti: ≤ 5×C |
| 1.4818 | X6CrNiSiNCe19-10 | - | S30415 | 0.06 | 19 | 10 | - | - | N: 0.16; Ce: 0.05. |
| 1.4828 | X15CrNiSi20-12 | 309 | - | < 0.2 | 20 | 12 | 2.0 | - | - |
| 1.4833 | X12CrNi23-13 | 309S | S30908 | < 0.08 | 23 | 13 | < 0.75 | - | - |
| 1.4872 | X25CrMnNiN25-9-7 | - | - | 0.25 | 25 | 7 | - | 9 | - |
| 1.4845 | X15CrNi25-21 | 310S | S31008 | < 0.1 | 25 | 20 | - | - | - |
| 1.4841 | X15CrNiSi25-21 | 314 | S31400 | < 0.15 | 25 | 20 | 1.8 | - | - |
| 1.4876 | X10NiCrAITi32-20 | "Alloy 800" | N08800 | < 0.12 | 21 | 32 | - | - | Al: 0.4; Ti: 0.4 |
| 1.4854 | X6NiCrSiNCe35-25 | "Alloy 353MA" | S35315 | 0.06 | 25 | 35 | - | - | N: 0.15; Ce: 0.06. |
| 1.4886 | X12NiCrSi35-16 | 330 | N08330 | < 0.15 | 18.5 | 35 | - | | - |
Type 309 and 310 are used in high temperature applications greater than 800 °C (1,500 °F).
Note: ferritic stainless steels do not retain strength at elevated temperatures and are not used when strength is required.
Austenitic stainless steel can be tested by nondestructive testing using the dye penetrant inspection method but not the magnetic particle inspection method. Eddy-current testing may also be used.

## Precipitation Hardening grade EN 1.4980

Grade EN 1.4980 (also known as A286) is not considered strictly as a heat resisting steel in standards, but this is popular grade for its combination of strength and corrosion resistance.
Typical composition :
| EN No. | EN designation | AISI/ASTM | UNS | C | Cr | Ni | Mo | Others |
| 1.4980 | X6NiCrTiMoVB25-15-2 | 660 | S66286 | 0.05 | 15 | 25 | 1.25 | V: 0.3; Ti: 2.0; B: 0.006. |
Minimum mechanical properties :
| Condition | Yield stress, min | Ultimate tensile strength, min | Elongation, min (%) |
| Solution treated and aged | 590 MPa (86 ksi) | 900 MPa (130 ksi) | 13 |
It is used for service temperatures up to 700 °C (1,300 °F) in applications such as:
* Aerospace (standardized in AMS 5731, AMS 5732, AMS 5737 and AMS 5525 standards),
* Industrial gas turbines,
* Automotive (turbo parts), etc.